Exhaust size?

After owning my TTI headers for quite some time, I am going to install them.The thing is I have a perfectly good 2 1/4 exhaust system on the car now and I would hate to have to spend 5 or 6 hundred to go to 2 1/2. So the question is how much HP/torque/power will I loose by leaving the 2 1/4 system on the car? It has a 340,auto,Air Gap,670 holly,electronic ignition,355's out back,shift kit, Dinomax mufflers. I do not know what cam as I have never taken the motor apart to find out. I think it is a little lumpier than stock but I do not know for sure. This is primarily a street car, may see a track one day if given the opportunity but I do like it to perform and it does quite well now. I would hate to put the headers on and see no gain if i leave the smaller pipes on.
What do you think? Again not a race car but I do like to keep the Chebby and Ford guys a little nervous.
Thanks